Genera by their very nature are expected to be monotypic and geographically and environmentally restricted at their origin, and most genera do not endure past their stage of first appearance. At the same time, those genera that do endure have a capacity to expand greatly in geographic range, environmental breadth, and species richness. Here we ask what it is that allows some genera and not others to survive past their inception. Using occurrence data from the Paleobiology Database, we find that initial geographic range has the strongest effect on survival, followed by environmental breadth, with the effect of species richness weaker on average. The effect of geographic range is strongest if measured as the distances spanned by the occurrences of a genus rather than the number of distinct areas in which a genus lives. We document substantial secular variation in selectivity of early survival. The most striking aspect of this variation is that survival is only weakly selective among genera that first appear during the Mesozoic. By following genera beyond their stage of first appearance, we find that selectivity with respect to all factors becomes systematically stronger as cohorts age and genera become more differentiated in range, breadth, and richness. This may help account for a previously identified statistical effect of genus age on the chances of survival.

Data that accurately capture the spatial structure of biodiversity are required for many paleobiological questions, from assessments of changing provinciality and the role of geographic ranges in extinction and originations, to estimates of global taxonomic or morphological diversity through time. Studies of temporal changes in diversity and global biogeographic patterns have attempted to overcome fossil sampling biases through sampling standardization protocols, but such approaches must ultimately be limited by available literature and museum collections. One approach to evaluating such limits is to compare results from the fossil record with models of past diversity patterns informed by modern relationships between diversity and climatic factors. Here we use present-day patterns for marine bivalves, combined with data on the geologic ages and distributions of extant taxa, to develop a model for Pliocene diversity patterns, which is then compared with diversity patterns retrieved from the literature as compiled by the Paleobiology Database (PaleoDB). The published Pliocene bivalve data (PaleoDB) lack the first-order spatial structure required to generate the modern biogeography within the time available (<3 Myr). Instead, the published data (raw and standardized) show global diversity maxima in the Tropical West Atlantic, followed closely by a peak in the cool-temperate East Atlantic. Either today's tropical West Pacific diversity peak, double that of any other tropical region, is a purely Pleistocene phenomenon—highly unlikely given the geologic ages of extant genera and the topology of molecular phylogenies—or the paleontological literature is such a distorted sample of tropical Pliocene diversity that current sampling standardization methods cannot compensate for existing biases. A rigorous understanding of large-scale spatial and temporal diversity patterns will require new approaches that can compensate for such strong bias, presumably by drawing more fully on our understanding of the factors that underlie the deployment of diversity today.

Mammalian carnivores are rarely incorporated in paleoenvironmental reconstructions, largely because of their rarity within the fossil record. However, multivariate statistical modeling can be successfully used to quantify specific anatomical features as environmental predictors. Here we explore morphological variability of the humerus in a closely related group of predators (Felidae) to investigate the relationship between morphometric descriptors and habitat categories. We analyze linear measurements of the humerus in three different morphometric combinations (log-transformed, size-free, and ratio), and explore four distinct ways of categorizing habitat adaptations. Open, Mixed, and Closed categories are defined according to criteria based on traditional descriptions of species, distributions, and biome occupancy. Extensive exploratory work is presented using linear discriminant analyses and several fossils are included to provide paleoecological reconstructions.

We found no significant differences in the predictive power of distinct morphometric descriptors or habitat criteria, although sample splitting into small and large cat guilds greatly improves the stability of the models. Significant insights emerge for three long-canine cats: Smilodon populator, Paramachairodus orientalis, and Dinofelis sp. from Olduvai Gorge (East Africa). S. populator and P. orientalis are both predicted to have been closed-habitat adapted taxa. The false “sabertooth” Dinofelis sp. from Olduvai Gorge is predicted to be adapted to mixed habitat. The application of felid humerus ecomorphology to the carnivoran record of Olduvai Gorge shows that the older stratigraphic levels (Bed I, 1.99–1.79 Ma) included a broader range of environments than Beds II or V, where there is an abundance of cats adapted to open environments.

Models presented here of shallow-marine siliciclastic deposition show that the widths of depth-defined regions differ markedly in response to sea-level change. These models add to recent studies that have emphasized the highly specific response of habitat area to sea-level change. Collectively, these studies indicate that a particular bathymetric zone on a particular margin may vary substantially in area during a sea-level change, while other such zones and margins may experience little or even opposite responses. In the models presented here, intermediate-depth and deep-water regions tend to show sinusoidal variations in width, with widening during relative falls in sea level and narrowing during relative rises. The shallow-water region displays markedly non-sinusoidal change and is consistently characterized by abrupt widening at the beginning of the highstand systems tract and an equally abrupt narrowing at the onset of sea-level fall at the beginning of the falling-stage systems tract. These onshore-offshore differences in how width and area change with sea level may explain why taxa in shallow-water settings tend to be more abundant, eurytopic, and widespread than those in deeper-water settings. Likewise, these models suggest that the evolution of novelty in nearshore habitats may be a response to wide variation in shallow-marine area during sea-level change.

Preservation in the fossil record is never perfect in the sense that we cannot sample all individuals of a given population in time and space. Incomplete detection (i.e., preservation and modern-day sampling of fossils) often affects estimates of other paleobiological parameters of interest, such as occupancy and turnover. Here, I simultaneously model the occupancy and detection probability of taxa, teasing apart the zeros in data that reflect true absences and those that imply non-detection of taxa that were actually present in the space and time of interest. Occupancy modeling, an approach first developed in population ecology, can easily incorporate covariates of interest, such as sampling effort and habitat variables. I use a data set of brachiopod taxa from the Paleozoic to illustrate the utility of this approach for paleontological questions. I demonstrate a range of models, including those that allow colonization between time intervals and those that incorporate facies as site covariates. I also suggest how future data collection can be improved so that process- and sampling-oriented approaches such as occupancy modeling can be applied with ease to paleobiological settings to answer important paleoecological and evolutionary questions.

Mean adult body mass of mammal taxa is a fundamental ecological variable. Variability in the distributions of body masses of a mammal fauna suggest variability in habitat structure. Mammal remains from the Marmes archaeological site in southeastern Washington State date between 13,200 and 10,400 b.p., during the Pleistocene–Holocene transition (PHT). Known environmental history prompts the expectations that the Marmes PHT mammal remains should represent greater species richness and a larger array of body-mass sizes than modern faunas in the Marmes locale and in open shrub-steppe habitats, and lower species richness and a smaller array of body-mass sizes than modern faunas in closed forest habitats; species richness and the array of body-mass sizes should be similar to that for a mixed habitat of cool shrub-steppe with scattered conifers. The Marmes PHT cenogram meets these expectations. Body-mass clumps displayed by the Marmes PHT mammal fauna fall between those of closed forests and open shrub-steppe habitats in terms of clump richness and breadth, and in terms of gap width. Marmes PHT body-mass clumps are very similar to those for the mixed habitat. Cenograms and body-mass clumps confirm conclusions drawn 40 years ago that the Marmes PHT habitat was much like that of today but cooler and with more plant biomass and greater structural diversity than today.

Previous genetic studies of extant planktonic foraminifera have provided evidence that the traditional, strictly morphological definition of species in these organisms underestimates their biodiversity. Here, we report the first case where this pattern is reversed. The modern (sub)tropical species plexus Globigerinoides sacculifer is characterized by large morphological variability, which has led to the proliferation of taxonomic names attributed to morphological end-members within the plexus. In order to clarify the taxonomic status of its morphotypes and to investigate the genetic connectivity among its currently partly disjunct (sub)tropical populations, we carried out a global survey of two ribosomal RNA regions (SSU and ITS-1) in all recent morphotypes of the plexus collected throughout (sub)tropical surface waters of the global ocean. Unexpectedly, we find an extremely reduced genetic variation within the plexus and no correlation between genetic and morphological divergence, suggesting taxonomical overinterpretation. The genetic homogeneity within the morphospecies is unexpected, considering its partly disjunct range in the (sub)tropical Atlantic and Indo-Pacific and its old age (early Miocene). A sequence variant in the rapidly evolving ITS-1 region indicates the existence of an exclusively Atlantic haplotype, which suggests an episode of relatively recent (last glacial) isolation, followed by subsequent resumption of unidirectional gene flow from the Indo-Pacific into the Atlantic. This is the first example in planktonic foraminifera where the morphological variability in a morphospecies exceeds its rDNA genetic variability. Such evidence for inconsistent scaling of morphological and genetic diversity in planktonic foraminifera could complicate the interpretation of evolutionary patterns in their fossil record.

Fossil bivalve shells are well-suited for landmark/semilandmark morphometric analysis because they preserve both traces of the internal anatomy and the whole shell outline. Utilizing landmarks and semilandmarks, we have characterized internal and external shape variation in a monophyletic clade of Cenozoic New Zealand and Australian crassatellid bivalves, to test the contiguity in morphospace of species-level taxa and to quantitatively examine the “Concept of Independent Entities” of Yonge (1953). Thirteen species from two genera (Spissatella Finlay 1926 and Eucrassatella Iredale 1924) are investigated. Spissatella n. sp. C is confirmed as forming a contiguous group separate to S. trailli and S. clifdenensis. Shell outline and internal anatomy are found to covary in shape, refuting the “Concept of Independent Entities” in the study group.

In quantitative paleobiology, one common format for image-derived information is the closed two-dimensional outline curve. Before the end of the last century a great variety of morphometric tools were on offer to deal with this data type, including diverse analyses of distance from a central point, “eigenshape analysis” of a carefully normalized tangent angle function, and Procrustes analysis of configurations of sliding semilandmarks. For the special case of outline forms that are nominally bilaterally symmetric, this paper offers a hybrid approach that fuses the Procrustes toolkit with a variant of a much older method, analysis of radial distance according to angles out of a center. The synthesis represents each outline as a regularly resampled data set of radially aligned shape coordinates analogous to bilateral Procrustes shape coordinates. When the center is iteratively located in one particular way, this representation has the same Procrustes distance statistics as a conventional radial representation, while the formalism of shape coordinates permits symmetrization, localizability, and thin-plate spline visualizations. We explain the method and sketch some of its advantages and limitations in the course of an example involving 99 Baculites inornatus outlines spanning 90 m of deposits from a high-precision measured section in the Campanian-age Rosario Formation cropping out at Punta San Jose, Baja California. This sequence shows substantial changes of pole curvature over time, changes that may involve a punctuation event regarding hydrostatics, hydrodynamics, or defense against predators. A concluding comment deals with implications of the developments here for Baculites microevolution and for morphometrics.

Numerous environmental and intrinsic biotic factors have been sought to explain patterns in diversity and turnover. Using taxonomically vetted and sampling-standardized data sets of more than 50,000 taxonomic occurrences in the Paleobiology Database (PaleoDB) we tested whether habitat breadth predicts genus durations and diversity dynamics of marine Mesozoic bivalves, and whether this effect is independent of the well-known positive relationship between geographic range and longevity. We defined the habitat breadth of a genus as a function of its realized ranges in water depth, substrate type, and grain size of the substrate. Our analysis showed that mean values of extinction and origination rates are significantly higher for narrowly adapted genera compared to broadly adapted genera, with differences being evident in all analyzed stratigraphic intervals. Linear models showed that both geographic range and habitat breadth have an independent effect on genus durations and on diversity dynamics. These results reaffirm the role of geographic range and furthermore suggest that habitat breadth is an equally important key predictor of extinction risk and origination probability in Mesozoic marine bivalves. Habitat generalists, regardless of their geographic range, are generally less prone to extinction. Conversely, widely distributed genera that are more specialized may be more endangered than one would expect from their geographic range alone. Extinction rates tend to be higher for specialized genera in both background and mass extinctions, suggesting that wide habitat breadth universally buffers against extinction. The trajectories of origination rates through time differ from those of extinction rates. Whereas there is no pronounced ecological selectivity in origination in the Triassic and most of the Jurassic, Cretaceous origination rates are higher for specialized genera. This may best be explained by diversity-dependence. When diversity levels reach a critical point a further increase in diversity is achieved by elevated origination rates of more specialized forms.

Extinction in the fossil record is most often measured by the percentage of taxa (species, genera, families, etc.) that go extinct in a certain time interval. This is a measure of taxonomic loss, but previous work has indicated that taxonomic loss may be decoupled from the ecological effects of an extinction. To understand the role extinction plays in ecological change, extinction should also be measured in terms of loss of functional diversity. This study tests whether ecological changes increase correspondingly with taxonomic changes during the Late Ordovician M4/M5 extinction, the Ordovician/Silurian mass extinction, and the Late Devonian mass extinction. All three extinctions are evaluated with regional data sets from the eastern United States. Ecological effects are measured by classifying organisms into ecological lifestyles, which are groups based on ecological function rather than evolutionary history. The taxonomic and ecological effects of each extinction are evaluated with additive diversity partitioning, detrended correspondence analysis, and relative abundance distributions. Although the largest taxonomic changes occur in the Ordovician/Silurian extinction, the largest ecological changes occur in the Late Devonian extinction. These results suggest that the ecological consequences of extinction need to be considered in addition to the taxonomic effects of extinction.

The regeneration abilities of crinoids not only are important to understanding crinoid ecology, but also can serve as the basis for assessing the pressure exerted on crinoids by predators both in the Recent and in the geologic past. This is especially true of regenerating arms, because arm loss, and subsequent regeneration, is thought to result from interactions with predators, primarily fish. However, the commonly used regeneration-based proxy for predation pressure (proportion of individuals with regenerating arms) does not provide a measure of the rate at which those events occurred. Here we present a method for reconstructing the arm-loss rate per individual, a more direct proxy of predation pressure. This metric accounts for differences in arm length, arm number, and branching pattern, features highly variable among taxa, among environments, and through geologic time. Normalizing for those characters permits the transformation of observed proportions of regenerating arms to rates that can be compared across samples of morphologically distinct crinoids. Applying this method to a Recent crinoid (Cenometra bella) reveals that this shallow-water comatulid loses arms at a rate of about once every ten days. The same approach reveals that Mississippian shallow-water crinoids (Rhodocrinites kirbyi) experienced arm loss much less frequently, approximately once every 36 days, suggesting that predation pressure on crinoids today is greater than it was in the Mississippian.

One of the most enduring evolutionary metaphors is Van Valen's (1973) Red Queen. According to this metaphor, as one species in a community adapts by becoming better able to acquire and defend resources, species with which it interacts are adversely affected. If those other species do not continuously adapt to compensate for this biotically caused deterioration, they will be driven to extinction. Continuous adaptation of all species in a community prevents any single species from gaining a long-term advantage; this amounts to the Red Queen running in place. We have critically examined the assumptions on which the Red Queen metaphor was founded. We argue that the Red Queen embodies three demonstrably false assumptions: (1) evolutionary adaptation is continuous; (2) organisms are important agents of extinction; and (3) evolution is a zero-sum process in which living things divide up an unchanging quantity of resources. Changes in the selective regime need not always elicit adaptation, because most organisms function adequately under many “suboptimal” conditions and often compensate by demonstrating adaptive flexibility. Likewise, ecosystems are organized in such a way that they tend to be robust and capable of absorbing invasions and extinctions, at least up to a point. With a simple evolutionary game involving three species, we show that Red Queen dynamics (continuous adaptation by all interacting species) apply in only a very small minority of possible outcomes. Importantly, cooperation and facilitation among species enable competitors to increase ecosystem productivity and therefore to enlarge the pool and turnover of resources. The Red Queen reigns only under a few unusual circumstances.

Within conifers, active abscission of complete penultimate branch systems is not common and has been described mainly from juveniles. Here I present evidence for the abscission of penultimate branch systems within early so-called walchian conifers—trees with a plagiotropic branching pattern. The specimens studied originate from a middle Early Permian gymnosperm-dominated flora within the middle Clear Fork Group of north-central Texas. Complete branch systems of three walchian conifer morphotypes are preserved; all have pronounced swellings and smooth separation faces at their bases. The source plants grew in a streamside habitat under seasonally dry climatic conditions. The evolution of active branch abscission appears to correspond to an increase in the size of conifers, and this combination potentially contributed to the restructuring of conifer-rich late Paleozoic landscapes. Moreover, trees shedding branch systems and producing abundant litter have the potential to affect the fire regime, which is a factor of evolutionary importance because wildfires must have been a source of frequent biotic disturbance throughout the hyperoxic Early Permian.

Over the past quarter-century there has been considerable innovation in methods for assessing the tempo and mode of evolution in paleobiological data sets. The current literature of these methods centers on three competing hypotheses—stasis, random walk, and directional trend—corresponding to an increasing scaling of variance with time interval (unchanging, for stasis; linear, for random walk; quadratic, for trend). For applications to a single trait there are powerful methods for discriminating among these hypotheses; but for multivariate data sets, especially the very high-dimensional multivariate data arising in image-feature-based and morphometric studies, current statistical approaches appear to be of less help. This paper proves that in the limiting case of high-dimensional morphospaces, the principal component or principal coordinate ordination of every sufficiently lengthy isotropic random walk tends to the same geometrical shape, which is not that of an ellipsoid and for which the principal components or coordinates are not independent even though they are uncorrelated. Specifically, the “scatter” of PC1 against PC2 is just a parabolic curve. The quantitative characteristics of this specific shape are not described appropriately by the corresponding “covariance structure” or Gaussian model, and the discrepancy may be pertinent to much of the existing literature of methods for differentiating among those three models of evolutionary multivariate time series. From a close examination of this common geometry of the ideal random walk model as seen in its principal components, I suggest a test for stasis, along with a mixed model illustrated by a reanalysis of some data of Gunz et al., and a related test for directional trend. These comments are intended to apply to all high-dimensional morphospaces, not just those arising in geometric morphometrics. Applications of principal components in this context distort high-dimensional data in ways that have a tendency to mislead; but these distortions can be intercepted so that studies of tempo and mode can nevertheless proceed.

In modern ecosystems, regions of topographic heterogeneity, when compared with nearby topographically homogeneous regions, support high species densities of mammals and other groups. This biogeographic pattern could be explained by either greater diversification rates or greater accommodation of species in topographically complex regions. In this context, we assess the hypothesis that changes in landscape history have stimulated diversification in mammals. Landscape history includes tectonic and climatic processes that influence topographic complexity at regional scales. We evaluated the influence of changes in topographic complexity and climate on origination and extinction rates of rodents, the most diverse clade of mammals.

We compared the Neogene records of rodent diversity for three regions in North America. The Columbia Basin of the Pacific Northwest (Region 1) and the northern Rocky Mountains (Region 2) were tectonically active over much of the Cenozoic and are characterized by high topographic complexity today. The northern Great Plains (Region 3) have been tectonically quiescent, with low relief, throughout the Cenozoic. These three regions have distinctive geologic histories and substantial fossil records. All three regions showed significant changes in diversification and faunal composition over the Neogene. In the montane regions, originations and extinctions peaked at the onset and close, respectively, of the Miocene Climatic Optimum (17–14 Ma), with significant changes in faunal composition accompanying these episodes of diversification. In the Great Plains, rodents showed considerable turnover but infrequent diversification. Peak Neogene diversity in the Great Plains occurred during cooling after the Miocene Climatic Optimum. These histories suggest that climatic changes interacting with increasing topographic complexity intensify macroevolutionary processes. In addition, close tracking of diversity and fossil productivity with the stratigraphic record suggests either large-scale sampling biases or the mutual response of diversity and depositional processes to changes in landscape history.

Geochemical tools, including the analysis of stable isotopes from fossil mammals, are often used to infer regional climatic and environmental differences. We have further developed an oxygen isotope aridity index and used oxygen (δ18O) isotope values and carbon (δ13C) isotope values to assess regional climatic differences between the southeastern and southwestern United States during the Pleistocene. Using data collected from previously published studies, we assigned taxa to evaporation-sensitivity categories by quantifying the frequency and magnitude of aridity index values (i.e., an average taxon δ18O value minus a site specific proboscidean δ18O value). Antilocapridae, Camelidae, Equidae, and Cervidae were identified as evaporation-sensitive families, meaning that a majority of their water comes from the food they eat, thus indicating that they are more likely to capture changing climatic conditions. Bovidae, Tayassuidae, and Tapiridae were identified as less sensitive families, possibly because of increased or more variable drinking behavior. While it is difficult to tease out individual influences on δ18O values in tooth enamel, the use of an aridity index will provide a more in-depth look at relative aridity in the fossil record. Greater aridity index values in the Southwest suggest a drier climate than in the Southeast during the Pleistocene, and δ13C values suggest that diet does not determine evaporation sensitivity. The combination of more-positive δ13C values and the lack of forest indicator taxa in the Southwest suggest that landscapes were more open than in the Southeast. Inferred higher aridity in the Southwest may indicate that aridity or seasonal aridity/precipitation, not temperature or pCO2, was a greater driver of C4 abundance during the Pleistocene. Collectively, these data suggest that regional climatic and environmental interpretations can be improved by using an aridity index and a more detailed understanding of mammalian paleobiology.

The continuous fossil record of therocephalian therapsids (Eutheriodontia) across the Permo-Triassic boundary and their differential survivorship of the end-Permian extinction offer an exceptional deep-time perspective on vertebrate life-history evolution during episodes of large-scale ecological perturbation. To examine potential impacts of the extinction on body size evolution (e.g., “Lilliput” effects) and growth patterns, we investigated cranial sizes and limb bone histology in the therocephalian Moschorhinus kitchingi both before and after the end-Permian extinction, facilitated by analysis of thin-sections of 23 limb bones from an ontogenetic sample of ten individuals across the Permo-Triassic boundary. In general, early subadult Moschorhinus displayed propodial cortices with extensive woven- and parallel-fibered bone (PFB) with dense radial and reticular vascularization and a moderately thickened bone wall with few growth marks. The outer cortex of propodials and epipodials showed a transition to PFB and lamellar bone with longitudinally oriented canals in individuals interpreted as late subadults or adults (>80% largest size). Most elements displayed several (3+) growth marks, though growth marks were more faithfully recorded in the epipodials of Permian individuals. Pearson product-moment correlation tests were performed to examine the relationship between size and robusticity on growth proxies (% cortical vascularity, mean primary osteon diameter), but variation in histomorphology could not be explained by size alone. Variation in body size may be affected by differences in juvenile growth rate and duration, which are highly variable in environmentally stressed extant reptile species. Geologic stage was a more consistent predictor of cortical vascularity. We suggest that Permian and Triassic Moschorhinus exhibited differential rates of early skeletal growth, corroborating the hypothesis that increased environmental variability in the earliest Triassic was associated with rapid growth to a minimum body size requirement and, consequently, shortened developmental times.

Mollusks in general and ammonoids in particular are known to display a sometimes profound morphological intraspecific variability of their shell. Although this phenomenon is of greatest importance, it has rarely been investigated and quantified. It is especially crucial for taxonomy and incidentally for biodiversity analyses to account for it, because otherwise, the number of described species might exceed that of actual species within any group. Early ammonoids (Early Devonian, Paleozoic) typically suffer from this bias. For instance, most specimens from the same layer and the same region (e.g., the Erbenoceras beds of the Moroccan eastern Anti-Atlas studied here) differ morphologically from each other. Depending on the importance given to certain morphological characters, therefore, one could create a new species for almost every specimen. In this study, we measured nearly 100 such specimens from a restricted stratigraphic interval and quantified their intraspecific variability. There is a variable but strong overlap of the quantified shell characters at most ontogenetic stages, and only two species are here separated rather than the four previously recognized in Morocco. When ontogenetic trajectories of the Moroccan specimens are compared with coeval faunas from other regions (assigned to other species), a strong overlap between the morphospaces occupied by these taxa becomes apparent. The justification of some of these latter species is thus questionable even if their mean values in some conch parameters differ considerably from the mean values of the Moroccan species. Hence, the number of currently valid species of these loosely coiled early ammonoids is probably much too high. Extreme caution must therefore be taken when examining the diversity of groups in which the intraspecific variability is poorly known.

Ontogenetic sequence reconstruction is challenging particularly for extinct taxa because of when, where, and how fossils preserve. Different methods of reconstruction exist, but the effects of preservational bias, the applicability of size-independent methods, and the prevalence of sequence polymorphism (intraspecific variation) remain unexplored for paleontological data. Here I compare five different methods of ontogenetic sequence reconstruction and their effects on the detection of sequence polymorphism, using a large collection of the extinct vertebrates Microbrachis pelikani and Hyloplesion longicostatum. The postcranial ossification sequences presented here for those taxa are the first examples known for extinct lepospondyls. Sequences were reconstructed according to skull length, trunk length, increasing number of ontogenetic events, majority-rule consensus, and Ontogenetic Sequence Analysis (OSA). Results generally were in agreement, demonstrating that paleontological data may be used to robustly reconstruct developmental patterns. When reconstructing sequences based on fossils, size-based methods and OSA are more objective and less dependent on preservational bias than other techniques. Apart from the other methods, OSA also allows for statistical analysis of observed and predicted polymorphism. However, OSA requires a large sample size to yield meaningful results, and size-based methods are justified in paleontological studies when sample size is limited by poor preservation. Different methods of reconstruction detected different patterns of sequence polymorphism, although across all methods the magnitude of sequence variation for M. pelikani and H. longicostatum (1.3−3.4%) was within the lower range of values reported for extant vertebrates. Compared with other extinct and extant tetrapods, all sequence reconstruction methods consistently showed that M. pelikani and H. longicostatum exhibit advanced ossification of the pubis and delayed ossification of the scapula. However, the postcranial ossification sequences of these two taxa largely are congruent with those of other tetrapods, suggesting an underlying conservative ancestral pattern that evolved early in tetrapod history.